The long term objective of this research is the elucidation of sites of action and mechanisms of action of gonadal hormones in the regulation of socio-sexual behavior. Research is proposed in four main, inter-related areas: 1) sites and mechanism of action of progesterone (P) in the regulation of estrous responsiveness; 2) the role of pregestin receptors in the mediation of the priming effect of estradiol (E); 3) sites of androgen action in the regulation of masculine sexual behavior; and 4) sites of action of gonadal hormones in the regulation of social behavior. It is generally agreed that the principal sites of P action in the regulation of estrous responsiveness is the ventromedial nucleus of the hypothalamus (VMN); however, it is likely that there are other sites of P action that contribute to this function. Intracerebral implants of P and synthetic agonists and antagonists will be employed to provide a clear identification of such sites which, in turn, will provide the basis of future studies on cellular mechanisms hormone of action in the regulation of behavior. In a second line of investigation, local induction of progestin receptors in the VMN by local application of E and microassay by nuclear punch will be carried out in order to determine whether there is a consistent correlation between priming of estrous behavior and induction of progestin receptors in the VMN. In the third area of investigation, the hormone implant approach will be used to obtain a more precise characterization of the sites of action of androgen in the regulation of masculine sexual behavior. Although it is well known that gonadal hormones influence social behavior including aggressive responsiveness and social communication in both males and females, little is known regarding the site of action of these hormones. This problem will be investigated with a combination of hormone implant approaches in conjunction with studies on the neuroendocrine regulation of sexual behavior. Taken together, these studies should provide an enrichment of our understanding of neuroendocrine mechanisms underlying reproductive and social behavior in rats as well as in other mammals, including humans.